The field of the disclosure relates to coated crucibles for holding melted semiconductor material and, particularly, for use in preparing multicrystalline silicon ingots by a directional solidification process. Other aspects include methods for coating crucibles, methods for preparing silicon ingots and wafers, compositions for coating crucibles and silicon ingots and wafers with a low oxygen content.
Conventional photovoltaic cells, used for the production of solar energy, utilize multicrystalline silicon. Multicrystalline silicon is conventionally produced in a directional solidification (DS) process in which silicon is melted in a crucible and directionally solidified in a separate or in the same crucible. The solidification of the ingot is controlled such that molten silicon solidifies unidirectionally at the solidifying front of the casting. The multicrystalline silicon produced in such a manner is an agglomeration of crystal grains with the orientation of the grains being random relative to each other due to the high density of heterogeneous nucleation sites at the crucible wall. Once the multicrystalline ingot is formed, the ingot may be cut into blocks and further cut into wafers. Multicrystalline silicon is generally the preferred silicon source for photovoltaic cells rather than single crystal silicon due to its lower cost resulting from higher throughput rates, less labor-intensive operations and the reduced cost of supplies as compared to typical single crystal silicon production.
During and after solidification, the solidified ingot must be released from the crucible without causing cracking of the ingot. Conventional crucibles are constructed of formed and sintered silica or fused-silica (synonymously “quartz”), silicon nitride or graphite. Silicon nitride crucibles may be reused but are typically more expensive relative to other crucibles. Crucibles may be coated with various coating materials; however, these processes and the resulting coated crucibles have been found to be deficient.